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Application of 2D Electrical Resistivity Tomography to Engineering Projects: Three Case Studies Songklanakarin J. Sci. Technol. 37 (6), 675-681, Nov. - Dec. 2015 http://www.sjst.psu.ac.th Original Article Application of 2D electrical resistivity tomography to engineering projects: Three case studies Rungroj Arjwech1* and Mark E. Everett2 1 Department of Geotechnology, Faculty of Technology, Khon Kaen University, Mueang, Khon Kaen, 40002 Thailand. 2 Department of Geology and Geophysics, Texas A&M University, College Station, Texas. Received: 2 December 2014; Accepted: 1 July 2015 Abstract Electrical resistivity tomography (ERT) is a non-invasive geophysical method of primary interest for addressing subsur- face engineering problems. The method is based on the assumption that subsurface geological materials have significant resistivity contrasts that can be identified based on measurements on the surface. This paper presents three different case studies that have been carried out at different sites. The first case study visualizes the contrast between high resistivity zones of hard bedrocks and low resistivity zones of weathered rocks. Similar to the first case study, the second case study shows high resistivity contrasts that clearly distinguishes the shape of a footing located within the surrounding materials. The third case study shows no clear low resistivity zone that can be identified as a leaking zone. The 2D ERT survey method used in these three investigations has been shown to be useful as a cost-effective and rapid method to obtain wide area subsurface informa- tion that is relevant for subsurface engineering problems. Keywords: non-invasive geophysical methods, electrical resistivity tomography (ERT), unknown bridge foundation determination, seepage 1. Introduction maintenance of highways (Dahlin, 2001; Wightman and Jalinoos, 2003). In many instances, geophysical NDT Geophysics involves the use of non-invasive tech- methods enhance the reliability and speed, and also reduce niques to determine subsurface anomalies without having to the cost of a geotechnical investigation (Anderson et al., 2008). engage in destructive excavation (Barker, 1993). Non-destruc- Assessing and characterizing geotechnical conditions can tive testing (NDT) is defined as the evaluation of the proper- become complex and costly in the presence of obstacles such ties of a material, component or system without causing as difficult access, irregular terrain and ground conditions, or damage (Louis, 1995). In the last few decades, geophysical regulatory constraints. Results based on traditional methods NDT methods have been developed and increasingly applied such as penetration testing or direct sampling may be of for addressing engineering problems. As one example, trans- limited utility. Surface geophysical techniques can provide portation personnel have used geophysical NDT methods alternate, wide-area methods for subsurface characterization in assisting geotechnical site investigation, construction, and and information regarding relevant material properties (Rucker, 2006). Though geophysics is not a substitute for geotechnical boring or testing, it is often a very cost-effective and efficient means of constructing contiguous 2D and 3D * Corresponding author. images of the subsurface and determining in-situ bulk Email address: [email protected] properties (Anderson et al., 2008). 676 R. Arjwech & M. E. Everett / Songklanakarin J. Sci. Technol. 37 (6), 675-681, 2015 The electrical resistivity tomography (ERT) method is one of the most widely used near-surface geophysical survey I 1 1 1 1 V , (1) methods for civil engineering applications (Castilho and Maia, 2 r1 r 2 r 3 r 4 2008). The method has been used for mapping electrical where r is resistivity, while r , r r and r are distances of resistivity in two and three dimensions (Dahlin, 2001). 1 2, 3, 4 the potential electrodes P and P from the current electrodes Previous studies have demonstrated the use of the ERT 1 2 C and C , respectively. Equation 1 is valid if the ground has method for identification of bedrock structures (Hsu et al., 1 2 homogeneous resistivity. 2010; Chambers et al., 2013), cavities or sinkholes (Kaufmann In case of inhomogeneous ground, an apparent resis- et al., 2012; Gómez-Ortiz and Martín-Crespo, 2012), geo- tivity r is calculated from the relationship between the technical site investigation (Al-Fares W., 2011; Haile and a applied current and the potential difference for a particular Atsbaha, 2014), slope stability investigation (Marescot et al., electrode arrangement and spacing. It is defined by, 2008; Perrone et al., 2014), and unknown bridge foundation determination (Arjwech et al., 2013; Tucker et al., 2014). k V , (2) The ERT method may be used for various other a I purposes in subsurface engineering investigations. This where k is a geometric factor dependent on the electrode paper aims to further demonstrate the application of the ERT spacing, technique on a number of engineering problems. More specifically, the paper presents the results of 2D ERT that 2 k . (3) have been carried out by the authors in various projects 1 1 1 1 including investigating the subsurface geology of a building r r r r construction site, determining the depth of an unknown 1 2 3 4 bridge foundation, and determining seepage from the earthen The apparent resistivity clearly depends on the geo- embankments of a wastewater treatment pond system. metry of the electrode configuration. The best electrode configuration for a field survey depends on the sensitivity 2. Electrical Resistivity Survey of the resistivity meter, the background noise level, and the relative importance assigned by the geophysicist to depth The electrical resistivity technique is based on the of penetration and lateral resolution. Standard electrode con- assumption that subsurface geological materials exhibit a figurations used for 2D ERT surveys are Wenner, dipole- wide variability of resistivity values and that geological dipole, Wenner-Schlumberger, pole-pole, pole-dipole, and boundaries can be identified based on measurements of resis- equatorial dipole-dipole (Telford et al., 1990; Loke, 2000; tivity. If a target of interest has a sufficiently large electrical Kearey and Brooks, 2002; Dahlin and Zhou, 2004; Loke and resistivity contrast with respect to that of the surrounding Lane, 2004; Loke, 2010). material, it can be detected by surface measurements of voltage following the injection of current through pairs of 3. 2D Electrical Resistivity Tomography electrodes (Barker, 1993). The purpose of a resistivity survey is thus to deter- A 2D multi-electrode ERT survey may be carried out mine the distribution of underground resistivity from using a large number of electrodes connected to a multi-core measurements of potential difference, or voltage, made on the cable. The electrode cable is typically divided into sections of ground surface. An electric current I (amperes, A) is injected manageable length, which are then connected end-to-end. at electrode C1 and withdrawn at electrode C2 as shown in Electrodes connected to the cable take-outs are inserted into Figure 1, while two other electrodes P1 and P2 are used to the ground at a specified regular interval along a survey line. record the resulting potential difference ÄV (volt, V), A resistivity meter and electronic switching unit are used in conjunction with a user-programmed protocol to automati- cally measure ra in a pre-defined sequence of combinations of four electrodes. Efficient data acquisition is achieved by measuring several voltages simultaneously across multiple pairs of electrodes following a single injection of electric current (Loke, 2000; Bernard, 2003; Hiltunen and Roth, 2003; Loke, 2010). Figure 2 shows an example of electrode arrangement and measurement sequence for a 2D ERT survey. When the data acquisition is completed, data analysis is performed Figure 1. Two current (C1 and C2) and two potential (P1 and P2) electrodes in the standard configuration (Telford et al., using the RES2DINV (Loke, 2004) software, including 2D 1990). pseudo-section plotting, and inversion. The RES2DINV R. Arjwech & M. E. Everett / Songklanakarin J. Sci. Technol. 37 (6), 675-681, 2015 677 Figure 2. Arrangement of electrodes for a 2D survey and the sequence of measurements used to build up an apparent resistivity pseudo-section (Loke, 2000). inversion algorithm is described by Loke and Barker (1995; 2 show similar resistive zones and thicknesses (Figure 4). 1996) and Yang (1999) and is based on a smoothness- The prominent low-resistivity zone of <100 m in the near- constrained least squares approach (DeGroot-Hedlin and surface towards the west corresponds to location without Constable, 1990). exposed bedrock and hence is interpreted as a zone of weathered rock and top-soil. The high-resistivity zone >200 4. 2D ERT to Investigate Subsurface Geology at a Building m that is dominant at the east end of the profiles is inter- Construction Site preted as sandstone bedrock. The inversion results from the profiles ChRU3 and 4 4.1 Site description show near-surface high resistivity >200 m which is consis- tent with resistant sandstone bedrock exposed on the A nine-story building is planned to be constructed surface. The high resistivity zone thins toward the north and on a site that is situated on a ridge characterized by cuesta extends to a maximum of ~15 m in depth at the southern ends topography aligned East-West (Figure 3A). The terrain is of the profiles. A low resistivity zone close to the northern covered with trees and bushes, with numerous boulders and ends can be seen at lower depths in both sections. These bedrock also contributing to make access difficult for a resis- lower zones of resistivity <100 m are the ones imaged on the tivity survey. The bedrock is well exposed on the surface and orthogonal profiles ChRU1 and 2. The inversion results of delineates the rim of a sedimentary basin structure. the profiles ChRU5 and 6 show similar structures as found in profiles ChRU3 and 4 but thicker high-resistivity zones are 4.2 Methods evident. The objective of this study is to identify bedrock. ERT 4.4 Data verification data acquisition comprises six profiles (ChRU 1-6) using SYSCAL R1 Plus by IRIS Instrument.
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